A surprisingly hard chapter. Meet up with Eirika's group using some flying and magic users, kill Caellech, then go kill Valter, cautiously. If you have the time and patience along with a summoner, it is possible to endlessly summon warriors to wear down Vaulter's(or any other boss's) weapons. Grab the things hidden under the sand using a thief/flying unit combo.
Side notes for this chapter. When you first get second party (eirika or ephiriam and their respective allies) take them straight to the castle north of them and the villages. Worth the 3 moves I promise. Also if this new group is the first time you're seeing knoll, move him north and stick him in the little area right above the castle near by, he starts off with very little heal and defense and will not likely survive any actual fights. If you want to keep him alive, and he is worth it as a summoner, hide him here it will keep him out of harms way.
To begin with take anyone with a bow who is high enough level for the mission (usually you want higher ranking people like a sniper for this mission) take them to them up and to the right a bit. Move weaker units south away from the keep a few spaces. From there take a team of 2 or 3 more powerful units to the left, don't approach the boss until you've killed all of his group, wipe up the trash then attack the boss. If you have the legendary sword or spell you just received they make this a quick and easy fight. If not, a high level great knight should do the trick. This is one of the harder bosses so don't make light of him.
As you work your way across 2 things will happen, 1 a small group of allies will appear on the lower right hand side, and 2 Pegasus knights will start to fly in from the north mostly on the left hand side, this is why the bowman are for.
It's very important that you quickly move your new group upwards, otherwise they will be overwhelmed quickly by groups of enemies that follow them onto the map in a few turns. After you've killed the first boss move your troops from that group south to meet up with your new group. leave one behind to kill the shaman groups that will soon appear from the north. About the time the boss dies or a couple moves after shaman will start to poor down from the north, much like the Pegasus knights except this time they will favor the left. Also groups of Calvary will start to appear in the same place where the new group came from, meet your two groups in the middle and work your way down to kill them, REMEMBER: hug the left hand side and don't take knoll into the fighting.
Around this time you want to be moving your bowman and other units you left at the keep downwards together, use the forts to your advantage and try to keep stronger units out front followed by ranged units who can one shot the dragon riders. Likewise move your southern group right in similar steps. This will cause the massive amount of troops surrounding valker to leave him to fight your troops, be patient and let them come to you valker, like many bosses, will not move and the small army he has will leave his side easily if you let them. Don't be afraid to end a turn without moving at all.
Once you've dealt with the enemy except for the final boss, valker, you have ample time to look around through the sand for the handful of items hidden there. You don't have to worry about any more reinforcements and valker won't attack you unless you get in range.
IN THIS LEVEL YOU CAN FIND - a master seal in the south western castle near where your reinforcements arrive, as well as an assortment of upgrade items!
Recommended units for this level:
You want to have at least 2 people you can dedicate to bows at least temporarily. Warriors are great for this because they have large amounts of health and can be used more effectively in ground combat too. Given long bows they help take valker down a lot.
At least one combat magic user, best to use a sage especially if you want to use the legendary spell to fight the bosses. Though it's best not to go overboard with the casters here as high health/defense count for more than attack in this level!
It only takes one healer if you are using the divide and conquer route, use stronger units at the lower left moving right, with elixirs so they can self heal, and moving your other units downward with your healer. A bishop is Perfect for this task as it allows you to have attack and healing in one. Though it's important to dedicate this unit to healing and not combat, the ability to defend yourself helps.
If Joshua is higher level you can give him the legendary sword and use him against the first boss for a quick victory with minimal damage. Then leave him to deal with the shaman that come.
The enemy here uses very very few archers so go use flying units, they aren't slowed by the sand!
Finishing notes - Try to stock up on spears, bows, and high health and defense units. Sense your in the sand everyone moves about the same so don't worry about whether your men are on horses or not. Though the bulk of the enemy force rides on horses or flying mounts, so spears vs horses and bows vs flying and the majority of the enemy force will get torn apart. The main thing to remember in this level is be patient and don't panic. Besides the need to move your reinforcements quickly you can spend most the level letting the enemy come to you. After the first boss is dead spend your moves setting your men up for the enemy to run into you, slowly making progress to bait the into charging. Let them hit walls like generals, great knights, or hero's then take them out with higher hitting/faster units like swordsmen, sages, or berserkers.